1. Field of the Invention
Embodiments of the present disclosure relate to compensation circuits for current peaking reduction. More particularly, the present disclosure relates to compensation circuits for reducing current peaking in current controlled pulse-width modulation (PWM) circuits for notification appliances such as those used in fire alarm systems.
2. Discussion of Related Art
Visual notification appliances, e.g. warning lights, are often used within buildings in conjunction with audio warning alarms so that the hearing impaired can be alerted to emergency conditions such as a fire. Typically, the visual notification appliance includes a flashing bulb or strobe positioned within a reflector. The bulb receives power from a power supply in a control panel. This power supply is normally powered by the building's AC supply, but also provides battery backup to ensure that the visual notification appliance will have power in the event power to the building is disrupted.
Visual notification appliances are subject to light intensity requirements as specified in various standards, such as Underwriters Laboratories UL 1971 (as well as UL 1638), “Standard for Safety Signaling Devices for the Hearing Impaired,” and the National Fire Protection Association's NFPA 72, The National Fire Alarm Code, all of which are incorporated herein by reference in their entirety.
The flash bulb or strobe of a visual notification appliance may be made up of a high-intensity xenon flash tube, a reflector assembly, a transparent protective dome, an electronic control circuit, a terminal block and a housing to accommodate installation of the device to a wall or ceiling. In various embodiments, the strobe of a visual notification appliance is designed to disperse its light output in a hemispherical pattern. The light distribution must meet the stringent specifications for UL approval, and it typically must accurately flash at a specified rate, for example, once per second or at some other multiple. Strobes in the same viewing area typically must be synchronized, as a fast flash rate or several unsynchronized strobes at the normal rate could cause susceptible people to have epileptic seizures.
The required intensity of the strobe, measured in candela, is dependent on occupancy, location, and local and national codes, standards and guidelines. For example, a strobe that is in a sleeping area and is required to wake the occupants is required to output more candela than a strobe located in a hallway. Notification appliances may include visual notification elements and audio notification elements. In various embodiments, however, the visual notification elements may draw more current than the audio notification elements. Each visual notification appliance may draw between 3 W-6 W of power, depending on the intensity of the light being emitted. The intensity of light may range from 15 candela to 185 candela, for example.
These notification appliances are connected to one or more central panels to define a notification system. The panels are used to control and provide regulated power to the plurality of notification appliances which are seen by the panel as a constant DC load for a given output voltage. For example, notification appliances may be designed to behave as a DC current load (e.g. RMS to DC variation is approximately on the order of 10-20% while the AC current/switching current/current interruption behavior is approximately less than 6-8%). This may be because current peaking may culminate in the addition of unwanted surge current when a number of notification appliances are populated and synchronized.
Efforts may be made to reduce current surges when peaking occurs using a regulated power supply or other means, including mechanisms using soft-start current limitation in the loads (e.g., notification appliances) upon start-up, or using current limiting circuits during repetitive start-up, or slow-charging smoothing circuits through specific requirements of UL1971. Despite these efforts, significant current peaking may still occur early and unintentionally in the notification appliance circuits in its steady-state operation. This phenomenon may be specific to the nature of the PWM circuit despite the fact that they incorporate current regulation. Although current peaking in these circuits may lead to shorter turn-on times for the notification appliances, the remnant charge for each duty cycle must be discharged before the next PWM cycle occurs.
In traditional PWM regulators where current-mode power supplies regulate voltage output, slope compensation may be used to control current peaking relating to wide duty cycle variation. In this example, to maintain a constant average current independent of duty cycle, a compensation circuit may be used, whereby, with increasing duty cycle the current regulation threshold is decreased in a descending slope, which may be referred to as slope-compensation. However, this application may not be suitable for flash tube constant-current PWM regulators that only regulate current. As a result, any form of compensation may not only distort input current waveforms and remove regulator control, but may also affect the net amount of energy delivered to a discharge (load) capacitor on a cycle-per-cycle basis.
Unlike most boost topology circuits that regulate output supply voltage, strobe notification circuits do not regulate output voltage but rather store charge through a constant-current cycling process. While the input characteristics may behave as a DC load, the output voltage is charged up exponentially over a period of approximately 1 second. The resultant behavior of the charge time is defined for a boost-circuit as follows:t(on)=[(Vout−Vin)*t(off)]/Vin 
The output voltage in a strobe flash tube charge cycle may vary anywhere from its start-up voltage 16<Vin<33 Volts to a voltage that may vary anywhere from 140<Vout<320 Volts, depending on strobe flash tube energy requirements (e.g., Candela settings). This may result in a 20:1 variation in turn-on (ton) time when compared to a turn-off cycle (toff) that may vary 2:1. Because many PWM circuits are substantially constant-frequency devices, the high duty cycle variation combined with the somewhat lesser turn-off duty variation may push the PWM circuit to function in and out of non-continuous mode. Even when a PWM is well designed and toleranced for a given application where the duty cycle variation is high, there is still a possibility that with narrow dead-time (e.g., substantially no inductor cycling turn-on or turn-off) the magnetic remanence may maintain a residual flux that may end-up causing peak currents. For example, in the initial phases where the (tcycle=tturn-on+tturn-off) time is very high, dead time may become very minimal which may result in a build-up of magnetic flux that does not get fully discharged. This build up may affect efficiency and may also draw extra current that does not translate into extra output. Consequently, it may be desirable to implement a circuit to compensate for, or reduce, current peaking in notification appliances. Therefore, the implemented mechanisms may adjust the dead time by maintaining it quasi-constant by reducing the switching frequency lower during the start-up period.